Distance sensors are also known as optical sensing heads or contactless sensors and are disclosed, for example, in European Patent Application Publication Nos. 0 156 991 and 0 163 347. These distance sensors are utilized, for example, in coordinate measuring technology for measuring or scanning workpieces.
Most of the known distance sensors include an infrared laser diode for generating the measuring-light beam to generate a spot having the smallest possible dimensions on the workpiece to be measured. This measuring spot is then imaged on the detector via the imaging beam path aligned at an angle to the measuring beam path. The distance to the measuring object is determined by electronic means from the position of the measuring spot on the detector.
The process of setting up the distance sensor above the workpiece surface is also known as "teach in". When setting up the distance sensor in advance of an actual measuring operation, the operating person has a series of tasks to perform with respect to the coordinate measuring apparatus which include the following:
(a) The sensor must be brought to a predetermined distance to the test object on which work is to be performed so that the surface of the test object is approximately in the center of the measuring region of the sensor.
(b) The measuring spot generated by the sensor is positioned at the location on the test object provided for beginning the measurement.
(c) In addition, it must be made certain that no obstructions shade the imaging beam path of the sensor.
Distance sensors which have a light source emitting a non-visible spectral range for generating the measuring spot such as an infrared laser diode present problems in this regard. This is so because additional assisting means such as a so-called infrared viewer is necessary in order to make the measuring spot visible to the operating person so that the above-mentioned work can be carried out. However, even when the measuring spot projected from the sensor is visible, difficulties nonetheless remain with respect to carrying out the above-mentioned points (a) and (c), since it cannot be recognized for certain when and to what extent the imaging beam path is shaded in the upper surface of the workpiece or test object. Such shading can be produced by edges or other non-uniformities.